Wednesday, January 17, 2007

The Omnimax - Revisited

On a recent trip to Townsville in North Queensland, I took some time out to re-experience the OMNIMAX cinema at Reef HQ. Whilst IMAX boasts a 3 story screen, the image is flat. The key difference here with OMNIMAX is the domed screen that literally envelopes you in both the image [180 degrees horizontally and 125 degrees vertically] and the multi-channel soundtrack - talk about immersive...


This fact display is quite neat. Being a bit if tech head, I got quite a kick out of this visit and even spoke to the projectionist about the technology...


Another piece of display showing the speakers behind the screen. This model was neat as it clearly shows the elements that make up the OMNIMAX cinema...


Where conventional cinema uses 35mm film pulled vertically through the projector at 24 frames/sec, both IMAX and OMNIMAX use 70mm film fed through horizontally, not vertically.

Both systems were invented here in Australia, but for some reason IMAX tends to want to keep the projection system "top secret". This photo was simply taken through the glass display on the way in to the cinema, because here, they want you see "behind the scenes"...


Although this image is dark, you can see the frame work that makes up the dome behind the Acoustically Transparent screen fabric. I was surprised that this frame was actually visible during the presentation, but it was not distracting.

The earlier presentations used to start with back lighting the screen to show off the sound system. It is said that more people are aware of surround sound for a soundtrack in their homes than are aware of the surrounds in a professional cinema. The demo [no longer used as a pre-show demo] certainly made you aware of the sound system by playing back isolated sounds from each speaker that blended to create a truly amazing 3D sound scape...


Back To The Top

Sunday, January 07, 2007

Going Acoustically Transparent.

This page is still a work in progress. Even though I have built my Curved Acoustically Transparent screen, I am always seeking to improve what I have. The goal is to create perfect AV integration, giving a much more cinematic, feel where sound cues actually match the on-screen visuals...

Here is hoping my set up can look this good. I really like the idea of back lighting the screen as I feel this would make an excellent feature wall when the projector is not operating.

Updated 02/12/2010

A New Hope
One issue with AT screens and digital projectors is 'moire'. Because the pixels of the digital projector are proportionally fixed to the screen height for a given throw [my current screen is 949mm high / 1080 pixels = 0.88mm], there comes a point when the system simply does not work - IE, the holes are the same size as the pixels. Rotating the fabric can help, however it may not provide the total fix. The solution is to either make the pixels larger [increase image size] or find a product with smaller holes. I've taken the 2nd option due to the size of my cinema and the fact that the screen is already wall to wall.

The original fabric [left] has 5%/95% perf to surface area. The new fabric [right] has just 3%/97% perf to weave, making it better for my screen due to less moire and even better AT properties.
So now there is a new hope for even better picture and sound at my screen size. I was very impressed with the image when I projected onto a sample and I am looking forward to replacing my screen with this new fabric soon. Both fabric samples are the same white as my current screen. The point of the photo was to show the weave, not the screen gain.

NB: This is NOT the new SmX 4K product that I saw in the US, however it should perform close.

Updated 29/09/09

The 'why' About A Curved Screen
The goal of the curved screen is to achieve pincushion correction. When the amount of curve is set correctly, the straight lines will be straight as can be seen on this test pattern.

Side Masking and Speaker Positions

The diagrams below are revised plans for speaker positions and side masking. The side masking will only cover the screen into the 1.78:1 ratio. I have also lowered the LCR speakers so they will be closer to seated ear height when the system is fully installed. Again the baffle is broken into 3 panels. Like my earlier designs, this will allow me to "toe in" the L and R speakers...

Screen Plan with Side Masking

Normally the L and R speakers are placed "wider" than the screen, but this method really only suits Constant Image Width systems, not Constant Image Height systems like mine. So with CIH [and like a real cinema], the speakers are placed just inside the side masking.

The LCRs

The idea of placing the L and R speakers "closer" together is for 2 main reasons -

1. It is important that sound cues will still match the smaller Aspect Ratios of 1.78:1 and 1.33:1, where placing them too wide would create "cognitive dissonance" [where two sensory inputs conflict]. You don't want to be seeing the visuals in one location, whilst hearing the sound cues in another. This can disrupt your "suspension of disbelief" whilst watching a film.
2. Even when AT side masks are used, there is a possibility that the some sound will not be passed as no material is 100% transparent. Placing the speakers inside the masking prevents this from ever happening. Remember, the sound is already passing through the AT screen material first.

The speakers will be flush mounted, similar to how they were mounted both in the SRT and my previous "lounge room" installation. The difference will of course be that the baffle will now be behind the screen, not under it. The entire baffle will also be covered in sound absorbing material [29 - 400 foam] to prevent reflections from the rear of the screen back on to the baffle.

Below is the construction log [or is that construction BLOG?] with the most recent update now at the top. Keep watching as I will post more photos, images and updated text as this projects develops...


Taking Shape

Today [28.09.07] I installed both the front screen mask and the side pillars which add a new lever to the install. The front screen mask attaches by two plastic L brackets that attach to the MDF between the alloy frame and the rear of the mask.

Nice Curves!

Looking along the screen and you can see the curve on the horizontal plain. I have also titled the LCR speakers down slightly to compensate for their hight in reference to the seating position.


The 1st Screen Shot

The AT Screen is now installed

The screen rig was installed today [20/09/07] and the speakers positioned behind the AT fabric. I have not properly aligned the projector/lens to the screen yet, but did watch a small portion of video this afternoon. WOW, there really is no compromise to hearing the sound coming from the images. Note the the screen masking still needs to be added.

The Back of the Screen Rig

UPDATE 15 The screen frame is now attached to the rig with a single bolt top and bottom [passing right through the timber] which also allows for precision threaded adjustment.

Simple But Effective

This is to ensure the exact amount of curvature will be a picture perfect match with my lens. Tonight I was able to get the required 70mm without a problem and can go more [at least another 25mm] if needed.

UPDATE 14 Today was testing, but I did manage to successfully curve the screen frame - but it had to go in backwards. Whilst the frame flexed beautifully, the fabric sagged when the frame was installed the correct way round. Re-tensioning did not solve the problem, but reversing the frame kept the Acoustic Vision fabric tight.

The Now Curved Screen

The radius is quite large as the centre only needs to be 70mm deeper than the sides to correct the pincushion caused by my lens. I will need to change the mounting hardware because whilst I did get the desired depth at the top, I came in slightly under at the bottom. Today I simply used 100mm baton screws, but I really need bolts with washers to be able to pull the screen in a touch more and ensure even curvature top and bottom.

Whilst I was always planning to make side masking, I will now also have to make cover strips that will hide the frame. I was planning on making some sort of cover top and bottom anyway to complement the side maksing, but will now have to make side covers as well to maintain the correct Aspect Ratio of 2.37:1 when the side masking is open.

The Back Of The Rig

A quick shot of the rear of the screen mount showing the three LCR speaker cut outs and shelves. The bolts that hold the screen will be tightened from the rear or the rig trough the timber. I have not yet made the LCR sub-enclosures.

The Modular Rig Plan

UPDATE 13: I had kind of put this project on the back burner. Over the weekend, I did some testing of the fabric with HD video as a source and a 720 DLP at a friends. A make shift frame was assembled to allow the screen material to be attached.

Whist the testing was crude, the results were pretty spectacular. As I have mentioned before, installing an A.T. screen is more complex than simply placing the fabric in front of your speakers and this set up quickly showed why. The temporary set up needed many extra parts not possible at the time, but I was still very impressed with the results to the point where I want to get this installed ASAP...

UPDATE 12: The Screen is now ready for testing! I have arranged the use of a Real Time Analyser to measure the acoustic properties of the fabric.

The OzTheatre Screen

UPDATE 11: The Foam has arrived! Today I got the chance get a shot of it installed along with an LCR in place. I had to buy two full sheets, so got the off cuts resized as well. They will be ideal for the side panels I am planning on adding to the sides. Because the sheets are nested pairs, there is enough to make all the room treatments I need.

LCR Mounted In The Baffle

I just need to colour code the shelves. I might run these down to my board cutter and get him to laminate the edge.

UPDATE 10: The OzTS Screen frame is now assembled, and the A.T. fabric is attached. I quickly placed it in front of my LCRs whilst listening to a SACD and could not hear any difference. I will be hiring a RTA, so proper test results will be interesting.

UPDATE 9: The Convoluted Foam [29-400 grade] has been ordered. I am getting the sheets cut to 1100mm x 800mm so that they will have straight edges. Originally I was going to make a simple shelf for the LCR speakers, but have now decided to construct enclosures to house the LCR speakers in case there is any light leakage behind the screen.

UPDATE 8: The subject of SIDE MASKING is again a hot topic. In my last CIH set up (in my house prior to selling) I did have side masking [based on the US "GULL WING" design] but this will now need to be re-designed so that it does not affect the dispersion pattern of the speakers that will be behind the screen. In the last set up, the LCR speakers were under the screen, so this was not such a concern.

UPDATE 7: Back lighting the screen is probably more "wank" factor than anything else, but as I mentioned above does look cool as a feature in the theatre when the projector is not in operation.

A Blue Neon

I checked out those LED lights [below] - gimmicky at best - but have found a better alternative [pictured above] that is still fairly cost effective. The foam has also been ordered for the Baffle.

Its all starting to take shape now.

The Baffle Is Now Assembled

As you can see from the image above, the screen's baffle is now assembled. I have followed the THX guide lines of 150mm [6"] between the speaker's baffle [the speakers will be flush after the foam goes in] and the rear of the screen. At this stage, the screen will not be curved, so here is hoping that this all works out.

UPDATE 5: I have not really done anything further [bought the timbre for behind the baffle] to this project, but am thinking these lights might look OK for back lighting behind the AT screen.

LED Lights? [maybe not]

They're LED so will not get hot. They are supposed to come in single colours [I want blue], but if they come as pictured, then two sets [with red in the centre] should look quite good. These should light the LCRs up quite nice...

UPDATE 4: Today I picked up the Baffle parts. I might be able to begin assembly tomorrow.

UPDATE 3: I have just picked the black MDF frame for the surround of the AT screen's baffle. The baffle [below] should be ready late next week. I am also still deciding on how the side masking will be attached - "wing thing" or slide.

UPDATE 2: The final draft for the AT screen baffle. Hopefully I can take this to the board cutters this week.

UPDATE 1: My AT screen arrived and is sitting in the box ready to be unpacked and assembled. I am just finalizing my baffle design before I place my order with my board cutter.